An ink jet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes called "dot locations", "dot positions", or "pixels". Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Thermal inkjet print cartridges operate by rapidly heating a small volume of ink to cause the ink to vaporize and be ejected through one of a plurality of orifices so as to print a dot of ink on a recording medium, such as a sheet of paper. Typically, the orifices are arranged in one or more linear arrays in a nozzle member. The properly sequenced ejection of ink from each orifice causes characters or other images to be printed upon the paper as the printhead is moved relative to the paper. The paper is typically shifted each time the printhead has moved across the paper The thermal inkjet printer is fast and quiet, as only the ink strikes the paper. These printers produce high quality printing and can be made both compact and affordable.
An inkjet printhead generally includes: (1) ink channels to supply ink from an ink reservoir to each vaporization chamber proximate to an orifice; (2) a metal orifice plate or nozzle member in which the orifices are formed in the required pattern; and (3) a silicon substrate containing a series of thin film resistors, one resistor per vaporization chamber.
To print a single dot of ink, an electrical current from an external power supply is passed through a selected thin film resistor. The resistor is then heated, in turn superheating a thin layer of the adjacent ink within a vaporization chamber, causing explosive vaporization, and, consequently, causing a drop of ink to be ejected through an associated nozzle onto the paper.
A concern with inkjet printing is the sufficiency of ink flow to the paper or other print media. Print quality is a function of ink flow through the printhead. Too little ink on the paper or other media to be printed upon produces faded and hard-to-read documents.
In an inkjet printhead ink is fed from an ink reservoir integral to the printhead or an "off-axis" ink reservoir which feeds ink to the printhead via tubes connecting the printhead and reservoir. Ink is then fed to the various vaporization chambers either through an elongated hole formed in the center of the bottom of the substrate, "center feed", or around the outer edges of the substrate, "edge feed". In center feed the ink then flows through a central slot in the substrate into a central manifold area formed in a barrier layer between the substrate and a nozzle member, then into a plurality of ink channels, and finally into the various vaporization chambers. In edge feed ink from the ink reservoir flows around the outer edges of the substrate into the ink channels and finally into the vaporization chambers. In either center feed or edge feed, the flow path from the ink reservoir and the manifold inherently provides restrictions on ink flow to the firing chambers.
Air and other gas bubbles can cause major problems in ink delivery systems. Ink delivery systems are capable of releasing gasses and generating bubbles, thereby causing systems to get clogged and degraded by bubbles. In the design of a good ink delivery system, it is important that techniques for eliminating or reducing bubble problems be considered. Most fluids exposed to the atmosphere contain dissolved gases in amounts varying with the temperature. The amount of gas that a liquid can hold depends on temperature and pressure, but also depends on the extent of mixing between the gas and liquid and the opportunities the gas has had to escape.
Changes in atmospheric pressure normally can be neglected because atmospheric pressure stays fairly constant. However, temperature does change within an inkjet cartridge to make an appreciable difference in the amount of gas that can be contained in the ink. Bubbles have less tendency to originate at low temperatures, and their growth will also be slower. The colder a liquid, the less kinetic energy is available and the longer it takes to gather together the necessary energy at specific location where the bubble begins to form.
Most fluids exposed to the atmosphere contain dissolved gases in amounts proportional to the temperature of the fluid itself. The colder the fluid, the greater the capacity to absorb gases. If a fluid saturated with gas is heated, the dissolved gases are no longer in equilibrium and tend to diffuse out of solution. If nucleation seed sites are present along the surface containing the fluid or within the fluid, bubbles will form, and as the fluid temperature rises further, these bubbles grow larger.
Bubbles are not only made of air, but are also made of water vapor and vapors from other ink-vehicle constituents. However, the behavior of all liquids are similar, the hotter the liquid becomes, the less gas it can hold. Both gas release and vapor generation cause bubbles to start and grow as temperature rises. One can reasonably assume the gases inside the bubbles in a water-based ink are always saturated with water vapor. Thus, bubbles are made up both of gases, mostly air, and of ink vehicle vapor, mostly water. At room temperature, water vapor is an almost negligible part of the gas in a bubble. However, at 50.degree. C., the temperature at which an inkjet printhead might operate, water vapor adds importantly to the volume of a bubble. As the temperature rises, the water vapor content of the bubbles increases much more rapidly with temperature than does the air content.
The best conditions for bubble generation are the simultaneous presence of (1) generating or "seed" sites, (2) ink flow and (3) bubble accumulators. These three mechanisms work together to produce large bubbles that clog and stop flow in ink delivery systems. When air comes back out of solution as bubbles, it does so at preferential locations, or generation or nucleation sites. Bubbles like to start at edges and corners or at surface scratches, roughness, or imperfections. Very small bubbles tend to stick to the surfaces and resist floating or being swept along in a current of ink. When the bubbles get larger, they are more apt to break loose and move along. However, if the bubbles form in a corner or other out-of-the-way location, it is almost impossible to dislodge them by ink currents.
While bubbles may not start at gas generating sites when the ink is not flowing past those sites, when the ink is moving, the bubble generation site is exposed to a much larger volume of ink containing dissolved gas molecules. As ink flows past the gas generating site, gas molecules can be brought out of solution to form a bubble and grow; while if the ink was not flowing this would happen less rapidly.
The third contributor to bubble generation is the accumulator or bubble trap, which can be defined as any expansion and subsequent narrowing along an ink passage. This configuration amounts to a chamber on the ink flow path with an entrance and an exit. The average ink flow rate, in terms of volume ink per cross section of area per second, is smaller within the chamber than at the entrance or at the exit. The entrance edge of the chamber will act as a gas generating site because of its sharpness and because of the discontinuity of ink flow over the edge. Bubbles will be generated at this site, and when they become large enough they get moved along toward the exit duct until the exit duct is blocked. Then, unless the system can generate enough pressure to push the bubble through, the ink delivery system will become clogged and ink delivery will be shut down. Thus, the chamber allows bubbles to grow larger than the diameter of subsequent ink passageways which may then become blocked.
During the ink filling and priming process, bubbles are left behind in the print cartridge. Bubbles can interfere with printhead reliability by causing intermittent nozzle problems and local or even global starvation. An important aspect of bubble control is the design of the internal cartridge geometry. The most critical areas for the design is the area around the substrate, headland, manifold, standpipe, and filters. The goals are to minimize dead spaces, streamline the geometry for fluid flow to avoid trapping bubbles during initial priming and to provide a clear path to allow for buoyancy to maximize the easy escape of bubbles from the printhead area into the ink manifold and then to float through standpipe and into filter area. Accordingly, a printhead design to be more tolerant of existing bubbles is desired.
Accordingly, there is a need for a printhead design to eliminate the residual bubbles left in the print cartridge after the ink filling and priming process.